The invention relates to an apparatus and method for inspection, quantitative testing, and/or topographic analysis of ends of optical fibers and optical connectors, and more particularly to a system and method utilizing an inexpensive interferometer including an electronically controlled parcentral, parfocal zoom objective lens to allow rapid changing of magnification so as to display an optimum number of available fringes of the interferogram without refocusing.
Use of optical fibers in various data communication systems and various optical devices has increased greatly over the past decade. Consequently, the need for measuring and testing the quality of optical fibers and optical connectors also has grown rapidly. The end faces of optical fibers and optical connectors must be of extremely high quality and very low deviation from optimum shape to prevent misalignments, air gaps, reflections or scattering of light at interfaces at which optical fibers need to be coupled in an optical fiber link. These imperfections are compounded by being summed with similar imperfections at all other fiber junctions in the system and ultimately can lead to greatly increased light attenuation, lower signal-to-noise ratios, and significantly lower system bandwidth.
Several low cost interferometer-based systems for inspecting optical fibers and optical connectors are available, including the CLEAVE-CHECK system and the CONNECT-CHEK system manufactured and marketed by Norland Products, Inc. of New Brunswick, N.J., the F-IM1 and F-IM2 interferometer systems for measuring the cleave angle and surface quality of an optical fiber end face marketed by Newport Corporation of Fountain Valley, Calif. and the Optispec Inspection system marketed by Micro Enterprises, Inc. of Norcross, Ga.
All of the above fiber inspection systems are less flexible and less accurate than desirable, and their utilization is more time consuming than desirable. The magnification of the above interferometer devices can be changed only by selecting a different fixed objective lens, and when this is done the interferometer must be refocused, and, usually the x,y alignment of the fiber or connector end surface also must be adjusted.
Although xy adjustment is required to truly center the sample after changing fixed objectives, this is very difficult to accomplish on the prior art devices, as only very crude adjustment techniques (if any) are provided for making such adjustments. The Newport and Micro Enterprise Systems inspection devices do not provide any accurate xy adjustment during their measurement sequences. On the Norland system a plate holding the sample is physically shifted by hand and locked down using two thumb screws when re-centering has been accomplished. Because of the foregoing lack of accurate xy adjustment capability, the prior art systems often do not use all of the available area on the screen of the display monitor to allow for such offsets, and thereby compromise data quality and visibility of fringes.
Each of the above mentioned operations is quite time-consuming. Inspection and measurement of various optical fiber or optical connector end faces usually necessitates use of various magnifications to first locate the sample, and then to obtain an optimum number of interference fringes within the field of view of the interferometer, if maximum accuracy of the measurement is to be achieved.
The economic realities of optical fiber and optical connector inspection systems dictate that they be relatively inexpensive. As a practical matter, fiber and/or connector inspection and measurement systems usually should cost less than approximately $10,000.00. This requirement prevents more expensive, sophisticated, commercially available interferometers (such as the Wyko 6000 described in U.S. Pat. No. 5,064,286, which costs roughly $50,000.00 or more) from being suitable for optical fiber and optical connector inspection applications. Zoom microscope objectives have been used in interferometers (such as the Wyko 6000) which are much more expensive and sophisticated than those used in fiber and connector inspection systems, but it is economically and technically unfeasible to adapt such interferometers for use in fiber and connector inspection systems.
Recent developments in the transmission and receiving ends of a fiber link have meant that the fiber and/or fiber connectors are likely to be the weakest link in achieving extremely high bandwidth fiber optic systems (especially for cable television systems and the like). This hurdle has led to the development of "super-polished" connectors, angled polished connectors and angled fiber cleaves. As tolerances are tightened and angles are purposely added, the task of measuring optical fibers and connectors becomes more difficult and far less tolerant of equipment error. Ensuring that two angled polished connectors will physically mate to within approximately a 1 micron tolerance is very difficult, but this is what is required to meet the needs of present high bandwidth fiber optic systems.
Furthermore, engineers now are placing far tighter tolerance requirements on "undercutting" or "overpolishing" of the end faces of optical fibers within optical connectors, wherein the fiber portion of an optical connector polishes more easily than the surrounding ferrule. This effect, however small, can produce an air gap between adjoining end faces, thereby reducing system bandwidth. The ability to measure both undercutting and equally undesirable (but less likely) protrusion of the optical fiber with respect to the connector end surface is becoming more essential as the technology of fiber optic links progresses.
It should be appreciated that approximately 70 fringes is the maximum number that can be readily resolved on an ordinary monitor. Using a conventional spherical FC/PC connector as an example, the same number of "available" fringes always will exist if the radius of curvature of the connector end surface within the field of view is below a certain value. In effect, more fringes actually are "available" than can be resolved on the monitor.
It should be appreciated that various standards and machines are used to polish connector ends, and the fixed objective systems of the prior art are never able to "see" the "optimum" number of the available fringes on all types of connector end surfaces to be tested. This effectively reduces the quality of the raw data of the prior art systems, which may lead to acceptance of lower quality optical connector end surfaces, and hence can lead to higher attenuation, lower bandwidth and lower signal to noise ratios in the optical systems made from such components.
It is difficult to overemphasize the importance of increased accuracy of measurement/testing of optical fiber and optical connector end surfaces in the overall effort to improve the quality of optical fiber interconnections and ultimately obtaining lower loss and higher bandwidth systems.
For at least the past decade, there has been a recognized but unmet need for a substantially improved but nevertheless inexpensive optical fiber and connector inspection and measurement system which provides substantially greater accuracy and substantially faster operation than has been achievable by use of the above mentioned commercially available fiber and/or connector inspection systems.